Method of manufacturing a molded article made from a macro defect free cementitious composition

ABSTRACT

A cementitious composition may include polyvinyl alcohol, high alumina cement, water, a metallic coagent, a peroxide crosslinking initiator, and an organic acid retardant. A molded article may be manufactured from the cementitious composition by preparing a hydrogel pre-polymer blend of saponified polyvinyl alcohol acetate (PVAA) with greater than or equal to approximately 85% saponified PVAA, and water, mixing the hydrogel pre-polymer blend with high alumina cement (HAC) using a high shear mixing process, mixing in a metallic coagent and a peroxide crosslinking initiator, mixing in an organic acid retardant, and hot press molding the mixture.

TECHNICAL FIELD

The present disclosure relates generally to macro-defect-free (MDF)cementitious compositions, and more particularly, to MDF cementitiouscompositions with moisture resistance.

BACKGROUND

Roadways, sidewalks, bridges, buildings, water ducts, reservoirs, andother infrastructure and structural components are often manufacturedfrom cementitious composites such as concrete. Concrete includes cementand other things, such as various aggregates and paste. Aggregatesinclude small materials such as sand, gravel or crushed stone. Often,the paste that holds the aggregates together is water and Portlandcement. Portland cement is a generic term for the most prevalent type ofcement. Cement typically makes up from 10% to 15% of the total mass ofconcrete. Portland cement is a type of hydraulic cement, which meansthat when water is added, a chemical reaction is started that causes thecement to harden and set, holding the aggregates together in a rocklikemixture—concrete. Before the concrete is allowed to harden, the concretemix is poured into a mold so that it will harden into the desired shape.The Portland cement is typically made from a combination of calcareousmaterial (usually limestone or other calcium carbonate-based materials)and argillaceous material (usually siliceous and aluminous mineralscontaining substantial amounts of clay-like components). A wide range ofchemicals are added to concrete that act as plasticizers, accelerators,retardants, dispersants, and water-reducing agents. Called admixtures,these additives can be used to increase the workability of a concretemixture, the strength of the concrete, the amount of time the concretewill take to harden and achieve full strength, and other desirableproperties. The proportions of the various raw materials that go intothe concrete must be carefully controlled and measured in order toobtain a finished product with the desired characteristics.

High strength cement-based materials such as macro-defect-free (MDF)cements are being developed for use in many applications that have notbeen possible with traditional cement and concrete technology. MDFrefers to the absence of relatively large voids or defects which areusually present in conventional mixed cement pastes because of entrappedair, inadequate dispersion, and porosity that develops as water soaksinto cement particles and aggregate and leaves behind voids. Such voidsand defects limit the strength of conventional Portland cement. MDFcement is a polymer-cement composite. The polymer and cement reactsynergistically to create a unique microstructure with distinctcharacteristics. The base polymer of the cementitious composite of MDFcement is a water-dissolvable polymer, such as polyvinyl alcohol. Highshear mixing and hot press molding processes are typically applied tothe mixture during production. MDF cements are characterized by veryhigh flexural strength and a high modulus of elasticity. Flexuralstrength, also known as modulus of rupture, bend strength, or fracturestrength, is a mechanical parameter for a material's ability to resistdeformation under load. Modulus of elasticity is a number that measuresan object or substance's resistance to being deformed elastically when aforce is applied to it. The relatively high flexural strength and highmodulus of elasticity of MDF cements are thought to be a result of theelimination of the majority of the voids that are in typicalcementitious composites caused by air entrapped during the mixing, andthe elimination of the majority of pores and capillaries that are formedwhen water is desiccated during cement hydration.

In spite of the promising mechanical properties of MDF cement materials,they have not been successfully commercialized because of poor waterresistance. The water-soluble polyvinyl alcohol base polymer continuesto be hydrophilic even after the initial curing reaction and thesubsequent water uptake lowers both the strength and modulus to lessthan half their original values. Many investigators have explored bothchemistry modifications to the MDF cement mixture and coatings orsurface treatments to render the material less hydrophilic.

European Patent EP0585998 B1 discloses a process for improving themoisture resistance through carbonation of a molded item. This processof carbonation requires immersion of an item in a water bath into whichcarbon dioxide gas is bubbled. This type of process may indeed renderthe surface of an item less susceptible to degradation but will notprovide long-term water resistance to a thick item.

The disclosed cementitious compositions are directed to overcoming oneor more of the problems set forth above and other problems of the priorart.

SUMMARY

In one aspect, the present disclosure is directed to a cementitiouscomposition. The cementitious composition may include polyvinyl alcohol,high alumina cement, water, a metallic coagent, and a peroxidecrosslinking initiator.

In another aspect, the present disclosure is directed to a moldedarticle made from a macro-defect-free (MDF) cementitious composition.The MDF cementitious composition may include polyvinyl alcohol, highalumina cement, water, a metallic coagent, and a peroxide crosslinkinginitiator.

In yet another aspect, the present disclosure is directed to a method ofmanufacturing a molded article made from a MDF cementitious composition.The method may include preparing a hydrogel pre-polymer blend ofsaponified polyvinyl alcohol acetate (PVAA) with greater than or equalto about 85% saponified PVAA, and water. The method may further includemixing the hydrogel pre-polymer blend with high alumina cement (HAC)using a high shear mixing process, mixing in a metallic coagent and aperoxide crosslinking initiator, and mixing in an organic acidretardant. The method may still further include hot press molding themixture at approximately 5 MPa pressure or greater at approximately 90degrees C. for approximately 30 minutes.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 illustrates an exemplary application of a cementitiouscomposition in accordance with various embodiments of this disclosure;

FIG. 2 is a flowchart illustrating exemplary steps for producing a bagcontaining a cementitious composition in accordance with variousembodiments of this disclosure;

FIG. 3 is a flowchart illustrating exemplary steps for fabricating amoldable product made from a cementitious composition in accordance withvarious embodiments of this disclosure; and

FIG. 4 is a chart showing comparisons between properties of examplecompositions described in this specification and known technologies.

DETAILED DESCRIPTION

The base polymer of high strength cementitious composites, and inparticular, macro-defect-free (MDF) cementitious composites, is awater-soluble, hydrophilic polymer such as polyvinyl alcohol (PVOH).High alumina cements (HACs) have been found to give superior strengthcompared to other cement types used in MDF cements. The surfaces of highalumina cement particles are abraded during mixing which releases bothcalcium ions and aluminum ions into solution to form Ca(OH)₂ and Al(OH)₃along with various other charged species (for example Ca(OH)₃ ⁻ andAl(OH)₄ ⁻ may be present). The alcohol side groups of the PVOH polymerthen chemically bind with the ionic species generated and form a highlycrosslinked ionomer—which reduces polymer chain flexibility and makesfor a rigid composite. As shown in FIG. 1, one example of an applicationof the MDF cement may be extruding the cement into a pothole or otherdefect in a road surface directly in front of a compactor 10. Thecompactor 10 may be equipped with a hopper 20 for receiving MDF cementin accordance with various implementations of this disclosure. A singlescrew extruder, twin screw extruder, or other mixing and extrudingmechanism 22 may also be mounted to the front of the compactor 10 toreceive the MDF cement from the hopper 20, mix the MDF cementitiouscomposite, and extrude the cement into the pothole or other desiredpoint of application. The high viscosity and rapid cure rate of theextruded cement allows for repairs to roads and other traffic surfaceswith a minimal amount of time during which traffic is disrupted.

However, the hydrophilic polymer, polyvinyl alcohol (PVOH) used in MDFcements may cause residual water uptake that lowers the strength andmodulus of elasticity of a finished product made from the MDF cementupon exposure to water or water vapor. Continued exposure of thefinished, molded product to water or water vapor after the MDF cementhas solidified may displace some of the ionic crosslinking and therebycompromise the strength and modulus of elasticity of the finishedproduct. In some applications utilizing conventional MDF cement, it hasbeen found that over half of the starting strength and two-thirds of thestarting modulus of elasticity of the cement may be lost after just afew weeks of water immersion.

The MDF cementitious compositions in accordance with various embodimentsof this disclosure are materials that have, as a considerable part oftheir strength, reliance upon ionic and covalent bonding associated withhydrated minerals and gels. Compared to traditional cements andconcretes, these MDF cements are characterized by a very high flexuralstrength (>100 MPa) and modulus of elasticity (>30 GPa). High shearmixing results in low amounts of defects that may be caused by voids,pores, or capillaries left behind in more traditional cement compositesfrom air bubbles or the evaporation of water. The high polymer contentand low water content in accordance with various implementations of thisdisclosure result in the MDF cementitious composite behaving like anelastomer during the mixing and curing processes.

Various additives and methods of manufacturing have been tried in orderto improve retention of the starting strength and modulus of elasticityafter water immersion or exposure to extended periods of high humidity.Some methods that have been explored include the addition ofaminosilanes, the addition of titanates, and the substitution ofphenolic resins for polyvinyl alcohol. None of these methods haveyielded an acceptable water resistance with minimal impact to processingand cost.

The novel MDF cement recipe of this disclosure is based on a mixture ofpolyvinyl alcohol polymer, water, and high alumina cement. Polyvinylalcohol (PVOH) is prepared by the saponification of polyvinyl acetate(PVAA). The polyvinyl alcohol that has typically been used in thecomposition of conventional MDF cement is a 78%-80% saponified polyvinylalcohol acetate (PVAA). Grades of PVAA with saponification levels belowabout 85% are referred to in this disclosure as “lower saponificationgrades”, and grades of PVAA with saponification levels equal to orgreater than 85% are referred to as “higher saponification grades”. Ithas been found that PVOH produced from the lower saponification gradesof PVAA provide a good balance of strength and processing time. However,a problem with the PVOH obtained from 78%-80% saponified PVAA is thatthese lower saponification grades are not readily available, and aresignificantly more expensive than higher saponification grades with 85%or higher saponified PVAA. The lower saponification grades of PVAA havehigher percentages of residual acetates, and the higher saponificationgrades of PVAA have lower percentages of residual acetates. For example,a 70% saponified grade of PVAA has approximately 30% residual acetates,and a 90% saponified grade of PVAA has approximately 10% residualacetates.

As the saponification percentage of PVAA increases, and the amount ofresidual acetates decreases, the available time for mixing and moldingof the MDF cement is significantly reduced. The alkalinity of the MDFcement mixture saponifies the residual acetate groups and generatesalcohol groups insitu. This chemical step slows the crosslinking rate ofthe PVOH with ionic species and the acetate counter ions also slows thecrosslinking rate. However, grades of PVAA with higher levels ofsaponification, such as higher saponification grades with greater thanor equal to 85% saponified PVAA, are less expensive and more readilyavailable than the lower saponification grades of 78%-80% saponifiedPVAA that have traditionally been used in MDF cement compositions.Nevertheless, these higher saponification grades of PVAA have not beenused in MDF cements because of their short processing times. Variousimplementations of the present disclosure allow for the use of thesemore readily available and less expensive higher saponification gradesof PVAA by the addition of an appropriate retarder to the recipe.Effective retarders may include organic acid retardants such as aceticacid, citric acid, tartartic acid, succinic acid, and polymeric acidssuch as polyacrylic acid. The addition of these organic acid retardantsto the MDF cement compositions may allow for adequate processing timewhen using grades of PVAA with saponification levels that are greaterthan or about equal to 85%.

In some preferred implementations of this disclosure, at least a portionof the water that is incorporated into the recipe for the improved MDFcement is pre-combined with the PVOH polymer to make a hydrogelelastomeric material that is conducive to high-shear mixing on atwo-roller mill or an internal mixer such as a BANBURY®-type internalmixer. It has been found that a polymer:water ratio in the range from100:75 to 100:150 provides an ideal hydrogel elastomer for rubber-likehigh-shear mixing. The PVOH polymer that yields a MDF cement with thebest properties for mixing and for molded articles generally has a highmolecular weight. For PVOH, the molecular weight is related to a 4%solution viscosity. A solution viscosity of 20 centipoise or greater isgenerally desired for the best combination of processing, strength, andmodulus of elasticity. This high molecular weight/high viscositycharacteristic results in a MDF cement that exhibits thixotropicproperties. In a preferred embodiment, a 4% solution viscosity of over40 centipoise represents a polymer with excellent processingcharacteristics and excellent mechanical properties of a molded article.With these high molecular weight polymers, a hydrogel elastomer may beprepared by mixing 100 parts of polymer and 75-150 parts of water andhomogenizing the blend by heating to 70° C.-90° C. for 16-24 hours. Insome embodiments, this polymer-water hydrogel may be prepared ahead ofwhen the final mixing and molding of a final article will occur. Thepolymer-water hydrogel elastomer may then be mixed with high aluminacement (HAC) using high-shear mixing, such as by employing a singlescrew extruder, a twin screw extruder, a continuous compounding mixersuch as the FARREL CONTINUOUS MIXER (FCM™), a kneader such as the BUSSKNEADER™ manufactured by Buss AG of Switzerland, a two-roll mill, or aninternal mixer such as the BANBURY®-type internal mixer.

The aforementioned organic acid retardants may be added during the earlystages of mixing of the MDF composition. If any additional water is tobe added (beyond that already combined in the polymer-water hydrogelelastomer), it may also be added near the beginning of the mixingsequence. Heat and pressure applied to the mixed MDF cement molds itinto a desired configuration and results in the material quickly curingto achieve adequate handling strength.

Example 1 below discloses a recipe for a MDF cement composition inaccordance with an embodiment of this disclosure, wherein the MDF cementcomposition includes an organic acid retardant, polyacrylic acid:

EXAMPLE 1 Polymer Selvol 540 100 parts by weight Water (120 partscombined 120 + 60 parts by weight with polymer, 60 parts added withcement) Cement-71% Secar 71 1500 parts by weight alumina HAC Retarder-Sokalan CP 10S 20 parts by weight polyacrylic acid

The recipe in Example 1 was mixed on a two-roll mill and molded in aheated hydraulic press (>5 MPa pressure) at ˜90° C. for >30 minutes. Thesample material was then postcured at approximately 90° C. for 48 hoursand then cut into flexural bars (˜4 mm thick×˜14 mm wide×˜150 mm long).The material initial properties were assessed and then samples wereimmersed in a stagnant water bath for various lengths of time and thechanges in flexural properties were measured. The results of thesemeasurements are shown in Table 1:

TABLE 1 No aging Initial strength (max), MPa 203 Initial modulus (avg),GPa 57 2 week water Strength (max), MPa 130 immersion Modulus (avg), GPa25 5 weeks Strength (max), MPa 127 immersion Modulus (avg), GPa 18 9weeks Strength (max), MPa 83 immersion Modulus (avg), GPa 15 12 weeksStrength (max), MPa 76 immersion Modulus (avg), GPa 13

As can be seen in Table 1, the embodiment including the addition of anorganic acid retardant to a MDF cement composition made with a highersaponification grade of PVAA (≧85% saponified) still has a significantreduction in flexural strength and modulus of elasticity upon long-termwater immersion, despite having an excellent initial strength.

In accordance with various preferred embodiments of the presentdisclosure, it has been unexpectedly discovered that the inclusion of ametallic coagent and peroxide crosslinking initiator can significantlyimprove the moisture resistance of products made from MDF cementitiouscompositions without the problems associated with adding titanates orzirconates, as attempted in some conventional MDF cementitiouscompositions. Examples of the metallic coagents that may be added to theMDF cementitious composition in accordance with various preferredembodiments of this disclosure may include zinc diacrylate (ZDA), zincmonoacrylate (ZMA), zinc dimethacrylate (ZDMA), calcium diacrylate(CDA), aluminum triacrylate, magnesium diacrylate, and other similarmetal-bound reactive monomers that may be reacted with peroxides.Peroxide crosslinking initiators that may find utility in such recipesinclude dicumyl peroxide (tradename DI-CUP,a,a′-bis(tert-butylperoxy)diisopropylbenzene (tradename VUL-CUP),1,1-di(tert-butylperoxy)-3,3,5-trimethylcyclohexane (tradename LUPEROX231), n-butyl-4,4-di(t-butylperoxy)valerate (tradename LUPEROX 230), andlower-temperature peroxides such as methyl ethyl ketone peroxide (MEKP)or hydrogen peroxide.

Example 2 below discloses a recipe prepared using this combination ofmetallic coagent and peroxide crosslinking initiator.

EXAMPLE 2 Polymer Selvol 540 100 parts by weight Water (120 partscombined 120 + 100 parts by weight with polymer, 100 parts added withcement) Cement-71% Secar 71 2000 parts by weight alumina HAC Processingaid Zinc Stearate 20 parts by weight Metallic Coagent- Dymalink 636 60parts by weight Calcium Diacrylate (SR636) Peroxide Di-Cup 40KE 20 partsby weight crosslinking initiator

The recipe in Example 2 was mixed on a two-roll mill and molded in aheated hydraulic press (>5 MPa pressure) at ˜90° C. for >30 minutes. Thesample material was then postcured at 90° C. for 24 hours and then cutinto flexural bars (˜4 mm thick×˜14 mm wide×˜150 mm long). The materialinitial properties were assessed and then samples were immersed in astagnant water bath for various lengths of time and the change inflexural properties were measured. This data is shown in Table 2.

TABLE 2 No aging Initial strength (max), MPa 121 Initial modulus (avg),GPa 56 2 week water Strength (max), MPa 92 immersion Modulus (avg), GPa40 4 weeks Strength (max), MPa 108 immersion Modulus (avg), GPa 36 8weeks Strength (max), MPa 105 immersion Modulus (avg), GPa 27 12 weeksStrength (max), MPa 94 immersion Modulus (avg), GPa 28

As can be seen in Table 2, the material of example 2 exhibits much lessreduction in both strength and modulus of elasticity compared to thematerial of example 1 that did not contain both the peroxide andmetallic coagent. Furthermore, this material exhibits better shelf lifeand processibility than formulations that contain titanates orzirconates as a moisture resistant additive.

Example 3 below combines the excellent shelf life benefits of adding anorganic acid retardant with the moisture resistant benefits of combinedperoxide and metallic coagent. The organic acid retardant may help withmoisture resistance as well.

EXAMPLE 3 Polymer Selvol 540 100 parts by weight Water (120 partscombined 120 + 80 parts by weight with polymer, 80 parts added withcement) Cement-71% Secar 71 2000 parts by weight alumina HAC Organicacid retarder Citric Acid (50% soln) 20 parts by weight MetallicCoagent- Dymalink 705 (SR705) 50 parts by weight Zinc DiacrylatePeroxide Di-Cup 40KE 10 parts by weight crosslinking agent

The recipe in Example 3 was mixed on a two-roll mill and molded in aheated hydraulic press (>5 MPa pressure) at ˜90° C. for >30 minutes. Thesample material was then postcured at 90° C. for 24 hours and then cutinto flexural bars (˜4 mm thick×˜14 mm wide×˜150 mm long). The materialinitial properties were assessed and then samples were immersed in astagnant water bath for various lengths of time and the change inflexural properties were measured. This data is shown in Table 3.

TABLE 3 No aging Initial strength (max), MPa 113 Initial modulus (avg),GPa 39 2 week Strength (max), MPa 138 water immersion Modulus (avg), GPa36 5 weeks Strength (max), MPa 107 immersion Modulus (avg), GPa 36 8weeks Strength (max), MPa 89 immersion Modulus (avg), GPa 34 12 weeksStrength (max), MPa 82 immersion Modulus (avg), GPa 35

As can be seen in Table 3, the combination of organic acid retardant,peroxide, and metallic coagent yields a MDF cement with very little lossin flexural strength or modulus of elasticity after long-term waterimmersion. Furthermore, because of the organic acid retardant, thisformulation has excellent shelf life (>1 hr) and flows well in a mold.

FIG. 4 shows how the disclosed examples 1 and 3 above compare with othertechnology known in the art. Glyoxal is known to crosslink PVOH and sohas been added in an attempt to slow down moisture ingression into thecured MDF cement. The addition of Glyoxal was found to help modestly,but did not give long-term modulus of elasticity or flexural strengthretention. Aminosilanes have also been added to MDF cement to react withthe PVOH and impact moisture resistance. The addition of aminosilaneswas found to be non-beneficial upon long-term immersion in water.Titanates have also been added to MDF cement in an attempt to improvemoisture resistance through crosslinking reactions with PVOH. Theaddition of titanates was found to be non-beneficial upon long-termimmersion in water.

The left-most portion of FIG. 4 shows results for Examples 1 and 3 ofthe present disclosure. The first vertical bar at the left-most portionof FIG. 4, representing the strength of the material sample from example1, illustrates a flexural strength in excess of 200 MPa. The strength ofthe sample decreased to about 130 MPa after 2 weeks of immersion inwater, and about 125 MPa after 4 weeks of immersion in water. Themodulus of elasticity of the sample from example 1 was initially about58 GPa, then about 23 GPa after 2 weeks of immersion, and about 18 GPaafter 4 weeks of immersion. The sample of example 3 had an initialflexural strength of about 120 MPa. The strength of the sample ofexample 3 then actually increased to about 140 MPa after 2 weeks ofimmersion in water, and decreased slightly to about 110 MPa after 4weeks of immersion in water. The modulus of elasticity of the sample ofexample 3 was initially about 39 GPa, and then decreased only slightlyto about 37 GPa after 2 weeks of immersion in water and after 4 weeks ofimmersion in water.

INDUSTRIAL APPLICABILITY

MDF cementitious compositions in accordance with various embodiments ofthe present disclosure may be used for structural purposes, much likefiberglass-reinforced thermoplastics and lightweight metals such asaluminum. In comparison to fiberglass-reinforced thermoplastics, themodulus of elasticity for MDF cements in accordance with variousembodiments of this disclosure is generally 2-10 times higher than themodulus of elasticity of fiberglass-reinforced thermoplastics. Thesemechanical characteristics allow for usage of the MDF cement inapplications where metals are currently in use and conversion to amolded material was previously prohibited because conventionalcementitious composites lacked sufficient flexural strength and modulusof elasticity. In comparison to lightweight metals such as aluminum(modulus of ˜70 GPa), the MDF cement materials of this disclosure havesimilar strength values but are generally less expensive. Therefore,die-cast aluminum parts or wrought aluminum fabrications may be replacedwith MDF cement pieces in order to lower product cost.

The MDF cementitious composition in accordance with various embodimentsof the present disclosure may be used to produce a finished, moldedproduct that has excellent flexural strength and modulus of elasticitywhen first produced, and retains the majority of these desirablemechanical characteristics even after immersion in water or exposure tohigh humidity for extended periods of time.

FIG. 3 is a flowchart showing an exemplary sequence of steps that may befollowed in producing a MDF cementitious composition in accordance withvarious embodiments of this disclosure. At step 320 a hydrogelpre-polymer blend may be prepared of saponified polyvinyl alcoholacetate (PVAA) with greater than or equal to a 85% saponification leveland water in a ratio of polymer:water that falls within the range fromapproximately 100:75 to 100:150. As discussed above, the highersaponified grades of PVAA, such as those used in step 320 with greaterthan or equal to approximately 85% saponification levels are inexpensiveand readily available when compared to the lower saponified grades ofPVAA that have traditionally been used in the production of MDF cement.The potential problem of short processing times when using the highersaponified grades of PVAA may be alleviated by the introduction of anorganic acid retardant into the mixture in order to allow enoughprocessing time.

At step 322, the hydrogel pre-polymer blend may be mixed with highalumina cement (HAC) using high shear mixing. Starting with the hydrogelpre-polymer blend of the desired higher saponified grades of PVAA andwater, rather than starting with the PVAA in powder form may alsoprovide significant advantages in that the hydrogel pre-polymer blend ismore conducive to high shear mixing. The hydrogel pre-polymer blendprovides an ideal hydrogel elastomer for rubber-like high shear mixing.Starting with the hydrogel pre-polymer blend may enhance the processingof the composition, including increasing the time between when themixing begins and when the MDF cementitious composite must be moldedinto its final form.

At step 324, a metallic coagent and a peroxide crosslinking initiatormay be mixed in to the composition. As discussed above, the metalliccoagent may be selected from a group of metallic acrylates and othersimilar metal-bound reactive monomers that may be reacted withperoxides. The peroxides used when the MDF composition is provided inbags may be peroxides with higher initiation temperatures than theperoxides used in other applications to ensure that no prematureinitiation of the crosslinking occurs before the bags are introducedinto a mixing mechanism. The crosslinking of the alcohol groups of thepolymer and metallic ions from the metallic coagent may overwhelm muchof the displacement of ionic crosslinking that would have otherwiseoccurred without the metallic coagent and peroxide as a result ofextended exposure of the finished, molded product to water. Thismechanism is thought to be the reason MDF cementitious compositions inaccordance with various embodiments of this disclosure exhibitsignificant increases in water resistance and retention of flexuralstrength and modulus of elasticity after immersion in water.

At step 326, an organic acid may also be mixed in as a retardant forimproving the shelf life of the composition, and improving moistureresistance. As discussed above, the organic acid retardant may beselected from a group of organic acids including acetic acid, citricacid, tartartic acid, succinic acid, and polymeric acids such aspolyacrylic acid. The organic acid retardant helps to counteract thereduced processing time that may result from using higher saponifiedgrades of PVAA, such as PVAA that is great than or equal to 85%saponified.

At step 328, the mixture may be molded into a desired configuration byhot pressing the mixture with pressures that are greater thanapproximately 5 MPa, and temperatures that are approximately 90 degreesC., ±30%. This hot pressing process may be maintained for approximately30 minutes, but depends on the thickness of the article as the materialis molded and cured into its final form. The final molded article may bepost-cured at step 330, with the temperatures being maintained atapproximately 90 degrees C.±30% for anywhere from several minutes toseveral days, depending on the application.

It will be apparent to those skilled in the art that variousmodifications and variations can be made to the disclosed cementitiouscomposition without departing from the scope of the disclosure. Otherembodiments of the composition will be apparent to those skilled in theart from consideration of the specification and practice of thedisclosure herein. It is intended that the specification and examples beconsidered as exemplary only, with a true scope of the disclosure beingindicated by the following claims and their equivalents.

What is claimed is:
 1. A method of manufacturing a molded article madefrom a MDF cementitious composition, the method comprising: preparing ahydrogel pre-polymer blend of saponified polyvinyl alcohol acetate(PVAA) with greater than or equal to about 85% saponified PVAA, andwater; mixing the hydrogel pre-polymer blend with high alumina cement(HAC) using a high shear mixing process; mixing in a metallic coagentand a peroxide crosslinking initiator, wherein the metallic coagent isselected from the group comprising zinc diacrylate (ZDA), zincmonoacrylate (ZMA), zinc dimethacrylate (ZDMA), calcium diacrylate(CDA), aluminum triacrylate, magnesium diacrylate, and other metal-boundreactive monomers that may be reacted with peroxides; mixing in anorganic acid retardant, wherein the organic acid retardant is selectedfrom the group comprising acetic acid, citric acid, tartartic acid,succinic acid and polymeric acids; and hot press molding the mixture. 2.The method of claim 1, further including: extruding the mixture to formthe molded article; and post curing the molded article at a temperatureof approximately 90 degrees C. for approximately 24 hours.
 3. The methodof claim 1, wherein the hydrogel pre-polymer blend is produced from aratio of polymer:water in the range from approximately 100 parts byweight polymer to 75-220 parts by weight water.
 4. The method of claim1, wherein the proportions of ingredients of the MDF cementitiouscomposition by weight comprise: 90-110 parts polyvinyl alcohol; 150-250parts water; 1200-3000 parts high alumina cement (HAC); 30-100 partsmetallic coagent; and 3-30 parts peroxide crosslinking agent.